1. Field of the Invention
The present invention relates to a ceramic (oxide) high temperature superconductor material, and more particularly, to a method of producing a Josephson element by using the ceramic superconductor material.
2. Description of the Related Art
The research into and development of ceramic high temperature superconductors is progressing rapidly, and this R&D has found that, when a ceramic superconductor material is used for a Josephson element of electronic devices or photodetectors, a high quality tunnel junction must be formed (Josephson junction).
As shown in FIG. 1, a conventional Josephson element having a superconductor-insulator-superconductor (SIS) tunnel junction structure comprises a substrate 1; a lower superconductor thin film 2 of, e.g., Nb or Pb; an insulator film 3 of, e.g., Al.sub.2 O.sub.3, MgO, SiO.sub.2 or oxide of Nb or Pb; an upper superconductor thin film 4 of, e.g., Nb or Pb; and terminal electrodes 5A and 5B of, e.g., Au, Ag. The lower superconductor thin film 2 is formed on the substrate 1 by a vapor deposition process or a sputtering process; the insulator film 3 having a thickness of less than 10 nm, to obtain the tunneling effect, is formed on the thin film 2 by a sputtering process or an oxidation process; and the upper superconductor thin film 4 is formed on the insulator film 3 by the vapor deposition process or the sputtering process used for the lower thin film 2. The Josephson element is used for an electronic device and a photodetector operating under an ultra low temperature of liquid helium (He).
When the metal material of the lower and upper superconductor thin films is replaced with a ceramic (oxide) superconductor material, a rough surface of the lower ceramic superconductor thin film causes pin-holes in the insulator film, which is several tens of nanometers thick and is made of MgO, Al.sub.2 O.sub.3 or SiO.sub.2. The pin-holes cause a leakage current to flow through the insulator film (i.e., a tunnel barrier), but if the insulator film is made thicker, to prevent the formation of pin-holes, the thicker film lowers the junction properties. The ceramic superconductor thin film can not be subjected to a surface oxidation treatment for the formation of the oxide film of the tunnel barrier applied on the metal material, but it is possible to treat the surface of the ceramic superconductor thin film with a CF.sub.4 gas plasma, to form the insulator film of fluoride. Nevertheless, such a fluoride insulator film still suffers from pin-holes formed therein. Where the Josephson element comprising the lower and upper ceramic superconductor films and the insulator film therebetween having pin-holes, as mentioned above, is used as a photodetector (photo-switching element), the photodetector can not realize a trouble-free operation, e.g., infrared rays can not be detected thereby, due to the flow of the leakage current.
Therefore, a Josephson element comprises the lower metal (Nb or Pb) superconductor thin layer and the upper ceramic superconductor. But, when used for the metal superconductor thin layer, the Josephson element must be cooled with liquid He, which invalidates the major advantage of using a ceramic superconductor material, (namely, use of liquid nitrogen (N.sub.2) instead of liquid He, or coolant not required).